The present invention relates to methods for producing toners for use in developing electrostatic images to be formed by an electrophotography, electrostatic recording process and other processes, and, more particularly, to a method which is capable of efficiently producing a toner having a narrow particle diameter distribution and causing a low degree of printing fog.
A toner to be used in an electrophotography apparatus, an electrostatic recording apparatus or another apparatus has been produced by various types of methods. To take an example, a classifying method (namely, crushing method) in which, after a colorant, a charge control agent, an offset preventing agent and other agents are uniformly dispersed in a thermoplastic resin by melt-kneading processing, the resultant resin was crushed to produce particles having a desired particle diameter. Another known example is the method (namely, polymerization method) in which droplets of a monomer composition containing a colorant and other additives are polymerized and optionally aggregated to directly produce toner particles.
In such the polymerization methods, characteristics of toner particles to be obtained depend on the uniformity of fine droplets of the monomer composition containing a colorant and other agents.
As a method for producing the monomer composition, a method of dispersing such that a viscosity change of more than 10 times an initial viscosity can be attained by using a media-type dispersing device has been proposed (Japanese Patent Laid-Open No. 75429/1994). However, even if a dispersion which has been subjected to such a large viscosity change is polymerized, the obtained toners have a wide particle diameter distribution, whereupon in a long period of operation, a fog on a light-sensitive material or paper can occur to deteriorate image characteristics.
An object of the present invention is to provide a method of efficiently producing a toner which has a narrow particle diameter distribution and generates minimal fog.
As a result of an extensive study by the present inventors for overcoming various problems in a method for producing a toner by a polymerization method, the present inventors have found that a toner which has a narrow particle diameter distribution and does not cause a fog can be produced by conducting a step of dispersing a colorant and other additives in a polymerizable monomer by using a media-type dispersing device with a specified rotation speed in a specified period of residence time and, being based on the above finding, have achieved the present invention.
Thus, according to the present invention, there is provided a method for producing a toner comprising the steps of: dispersing a colorant in a dispersion medium, essentially composed of a polymerizable monomer, by using a media-type dispersing device at a rate of the tip of a stirring body of 3 to 20 m/s with a residence time being 0.03 to 0.5 hour to obtain a monomer composition A; homogenizing the above-described composition A in an aqueous dispersion medium containing a dispersion stabilizer to generate droplets; and polymerizing the above-described droplets to obtain colored polymer particles.
Further, according to the present invention, there is provided a method for producing a toner comprising the steps of: dispersing a colorant in a dispersion medium (a), essentially composed of a polymerizable monomer, to obtain a monomer composition A; dispersing or dissolving a charge control agent in a dispersion medium (b), essentially composed of a polymerizable monomer, to obtain a monomer composition B; dispersing or dissolving a lubricant in a dispersion medium (c), essentially composed of a polymerizable monomer, to obtain a monomer composition C; dispersing or mixing the monomer composition A, the monomer composition B and the monomer composition C in a dispersion medium (d), essentially composed of a polymerizable monomer, to obtain a polymerizable composition D; homogenizing to form droplets of the above-described polymerizable composition D in an aqueous dispersion medium containing a dispersion stabilizer; and polymerizing the above-described droplets to obtain colored polymer particles.